1. Field of the Invention
The present invention relates to the art of electrical filters and specifically to a sampled data filter, employing bucket brigade technology, the response of which may be varied with the alteration of a stored reference function.
2. Description of the Prior Art
Bucket brigade delay line (BBDL) technology which allows information storage in analog form offers the means by which the theory that has been developed for the design of digital filters can be utilized to design sampled data analog filters without the need for analog-to-digital conversion, thus combining the advantages of digital signal processing with the speed and simplicity of analog circuitry. These techniques have been employed in the prior art to design recursive and transversal type filters. The response characteristics, i.e., center frequency and bandwidth, of the recursive filters depend on the circuit parameters of gain and BBDL clock frequency, both of which may be electrically controlled thus providing a programmable filter. However, the recursive structure gives rise to spurious responses that limit the filter's frequency range of operation. Techniques to minimize the spurious responses require an appreciable increase in circuitry and gain parameters with a concomitant increase in complexity and cost.
Prior art transversal filters utilizing BBDL technology have been designed with fixed and programmable response characteristics. Fixed response designs establish desired weighting at each tap by means of a suitable choice of storage capacitance values whereas the programmable designs utilize amplifiers at each tap, the transconductance of which is electrically altered to provide the required weights to achieve the desired frequency response. Stop band performance characteristics of these filters is dependent upon the relative accuracy that can be achieved in the tap weight implementations, which is limited by the transconductance control ability of the amplifiers. Additionally, the filter characteristics that may be programmed are limited by the transconductance ranges of the amplifiers utilized.
The present invention provides a programmable transversal filter wherein the tap weights are supplied by a stored reference function, thereby eliminating the limitation on the range of filter characteristics that may be realized that are imposed by the achievable relative tap weight accuracies and transconductance ranges of the prior art.